turtles-own

[ covid_positive?          ;; true if the individual currently has covid-19

  wearing_mask?            ;; true if the individual is wearing a mask

  remaining_immunity_weeks ;; weeks of immunity left for the individual

  infection_duration       ;; duration in weeks the individual has been infected

  individual_age          ;; age of the individual in weeks

  vaccinated? ]           ;; vaccination status

​

globals

[ percent_positive         ;; what percent of the population is covid-positive

  percent_immune           ;; what percent of the population is immune

  individual_lifespan      ;; expected lifespan of an individual

  reproduction_odds        ;; odds of an individual reproducing each tick

  environment_capacity     ;; carrying capacity of the environment

  immunity_period         ;; how long immunity lasts, in weeks

  vaccine_effectiveness_reduction ] ;; reduction in infection chance due to vaccination

​

;; initialize the model

to setup

clear-all

define_parameters

initialize_population

calculate_statistics

visualize_population

reset-ticks

end

​

;; create a set number of individuals with some initially covid-positive

to initialize_population

let num_vaccinated round((percent_vaccinated / 100) * population)

create-turtles population

  [ set shape "person" 

    setxy random-xcor random-ycor

    set individual_age random individual_lifespan

    set infection_duration 0

    set remaining_immunity_weeks 0

    set size 1.5  ;; makes individuals easier to spot

    regain_health

    set wearing_mask? random-float 100 < people-masked

    set vaccinated? false ] ;; Ensure all turtles start as not vaccinated

ask n-of 10 turtles [ become_infected ]

ask n-of num_vaccinated turtles [ vaccinate ]

end

​

;; procedures to change the state of health of the individuals

to become_infected

set covid_positive? true

set remaining_immunity_weeks 0

end

​

to regain_health

set covid_positive? false

set remaining_immunity_weeks 0

set infection_duration 0

end

​

to develop_immunity

set covid_positive? false

set infection_duration 0

set remaining_immunity_weeks immunity_period

end

​

;; set up model constants

to define_parameters

set individual_lifespan 50 * 52      ;; lifespan in weeks

set environment_capacity 300

set reproduction_odds 0.01           ;; 1% reproduction chance

set immunity_period 52               ;; immunity lasts one year

set social-distancing 10             ;; intial level of social distancing

set vaccine_effectiveness_reduction 0.18  ;; 82% reduction in infection chance due to vaccination

end

​

;; main model loop

to go

ask turtles [

  age_individual

  random_move

  if covid_positive? [ resolve_infection ]

  ifelse covid_positive? [ transmit_virus ] [ reproduce ]

]

calculate_statistics

visualize_population

tick

end

​

;; update statistics for the population

to calculate_statistics

if count turtles > 0

  [ set percent_positive (count turtles with [ covid_positive? ] / count turtles) * 100

    set percent_immune (count turtles with [ is_immune? ] / count turtles) * 100 ]

end

​

;; update visuals based on health status

to visualize_population

ask turtles

[ set color ifelse-value covid_positive? [ red ] [ ifelse-value is_immune? [ grey ] [ ifelse-value vaccinated? [ blue ] [ green ] ] ] ]

end

​

;; procedures to manage individual aging and health status

to age_individual

set individual_age individual_age + 1

if individual_age > individual_lifespan [ die ]

if is_immune? [ set remaining_immunity_weeks remaining_immunity_weeks - 1 ]

if covid_positive? [ set infection_duration infection_duration + 1 ]

end

​

to random_move ;; makes individuals move randomly

let move-distance 1

ifelse any? other turtles in-radius 3

[

  let distance-factor (100 - social-distancing) / 100

  set move-distance move-distance * distance-factor

]

[

  set move-distance 1

]

rt random 100

lt random 100

fd move-distance

end

​

;; handling virus transmission with mask and season modifiers

to transmit_virus

ask turtles [ if mask-effectiveness < 0.1 [set mask-effectiveness 0.1] ]

ask other turtles-here with [ not covid_positive? and not is_immune? ]

[

  let effective_transmission_modifier transmission_modifier

  if wearing_mask? [

    set effective_transmission_modifier effective_transmission_modifier * mask-effectiveness

  ]

  if vaccinated? [

    set effective_transmission_modifier effective_transmission_modifier * vaccine_effectiveness_reduction

  ]

  if random-float 100 < (infectiousness * effective_transmission_modifier) / (ifelse-value wearing_mask? [mask-effectiveness] [1]) [

    become_infected

  ]

]

end

​

;; resolution of infection, individuals recover or perish

to resolve_infection

if infection_duration > duration

  [ ifelse random-float 100 < recovery-rate

    [ develop_immunity ]

    [ die ] ]

end

​

;; reproduction logic based on environment capacity

to reproduce

if count turtles < environment_capacity and random-float 100 < reproduction_odds

  [ hatch 1

    [ set individual_age 1

      lt 45 fd 1

      regain_health ] ]

end

​

to-report transmission_modifier

let modifier 1

ifelse current-season = "Winter" [

  set modifier 1.56

] [

  set modifier 0.54

]

report modifier

end

​

;; report immunity status

to-report is_immune?

report remaining_immunity_weeks > 0

end

​

to vaccinate

set vaccinated? true

end

​

;; ensure parameters are defined for the user interface elements

to startup

define_parameters ;; sets up constants used in the interface

end

​